docs: def _process_omim2disease(self, limit=None):
          """
          This method maps the KEGG disease IDs to
          the corresponding OMIM disease IDs.
          Currently this only maps KEGG diseases and OMIM diseases that are 1:1.
  
          Triples created:
          <kegg_disease_id> is a class
          <omim_disease_id> is a class
          <kegg_disease_id> hasXref <omim_disease_id>
          :param limit:
  
          :return:
  
          """
  
          LOG.info("Processing 1:1 KEGG disease to OMIM disease mappings")
          if self.test_mode:
              graph = self.testgraph
          else:
              graph = self.graph
          line_counter = 0
          model = Model(graph)
          raw = '/'.join((self.rawdir, self.files['omim2disease']['file']))
          with open(raw, 'r', encoding="iso-8859-1") as csvfile:
              filereader = csv.reader(csvfile, delimiter='\t', quotechar='\"')
              for row in filereader:
                  (omim_disease_id, kegg_disease_id, link_type) = row
  
                  kegg_disease_id = 'KEGG-' + kegg_disease_id.strip()
                  omim_disease_id = re.sub(r'omim', 'OMIM', omim_disease_id)
  
                  # Create hash for the links from OMIM ID -> KEGG ID
                  if omim_disease_id not in self.omim_disease_hash:
                      self.omim_disease_hash[omim_disease_id] = [kegg_disease_id]
                  else:
                      self.omim_disease_hash[omim_disease_id].append(kegg_disease_id)
  
                  # Create hash for the links from KEGG ID -> OMIM ID
                  if kegg_disease_id not in self.kegg_disease_hash:
                      self.kegg_disease_hash[kegg_disease_id] = [omim_disease_id]
                  else:
                      self.kegg_disease_hash[kegg_disease_id].append(omim_disease_id)
  
          # Now process the disease hashes
          # and only pass 1:1 omim disease:KEGG disease entries.
          for omim_disease_id in self.omim_disease_hash:
              if self.test_mode and omim_disease_id not in self.test_ids['disease']:
                  continue
  
              if (not self.test_mode) and (limit is not None and line_counter > limit):
                  break
              line_counter += 1
  
              if len(self.omim_disease_hash[omim_disease_id]) == 1:
                  kegg_disease_id = ''.join(self.omim_disease_hash.get(omim_disease_id))
                  if len(self.kegg_disease_hash[kegg_disease_id]) == 1:
                      # add ids, and deal with the labels separately
                      model.addClassToGraph(kegg_disease_id, None)
                      model.addClassToGraph(omim_disease_id, None)
                      # TODO is this safe?
                      model.addEquivalentClass(kegg_disease_id, omim_disease_id)
              else:
                  pass
                  # gu.addXref(g, omim_disease_id, kegg_disease_id)
                  # TODO add xrefs if >1:1 mapping?
  
          LOG.info("Done with KEGG disease to OMIM disease mappings.")
          return